OTHER CONCENTRATION UNITS

Yves Alarie, Ph.D Professor Emeritus University ofPittsburgh,USA

A. PARTIAL PRESSURE

Fraction (Volume/Volume Composition) of the total pressure of a mixture exerted by a component of the mixture.

For example, at sea level, 1 atmosphere (760 mm Hg, 760 torrs, 1.1013 bars, 1.03 kg/cm2, 14.7 p.s.i.a. or 101.325 kPa and yes you were under the impression that SI units were going to simplify your life, fat chance!), if we know the volume composition of each component in the mixture, we can calculate the partial pressure of each. Assuming the following % composition for air:

N2 = 78.6% = 597 mm Hg Partial Pressure

O2 = 20.8% = 159 mm Hg Partial Pressure

CO2 = 0.04% = 0.3 mm Hg Partial Pressure

H2O = 0.5% = 3.8 mm Hg Partial Pressure

TOTAL 100% = 760 mm Hg Total Pressure

The volume composition can be in % or ppm, see above, since both are volume/volume unit. Therefore, 0.1% or 1,000 ppm = 0.76 mm Hg.

B. pV = nRTIf the concentration is given as moles/Liter (n/V), the partial pressure p (in mm Hg) can be obtained using the value of the gas constant R as 62 and the temperature (T) in Kelvin. Seldom used in toxicology but certainly appropriate.

C. HENRY’S LAW AND SOLUBILITY COEFFICIENTThe amount (moles or grams) of a gas or vapor dissolved in

a liquid is directly proportional to the partial pressure of the gas in the gas (air) phase above the liquid. At equilibrium the partial pressure (in mm Hg) in both phases is equal, but the amount, in moles or grams, will vary between the two phases according to each gas. This ratio, S, is called the solubility coefficient and is given for a particular temperature.

S = Concentration in moles (or grams) of a gas in a liquid/concentration (same units) of a gas in air phase.

gas C or air C

liquid C = S

Phases Partial Pressure

(mm Hg)

Concentration

(mg/L)

Solubility

Coefficient

Air or Gas 100 50 100

Liquid 100 5,000

Cl/Cg = 1/100 = 0.01 = Low: Nitrogen,

Helium, Oxygen

Cl/Cg = 150/100 = 0.15 = Moderate: CO2,

N2O

Cl/Cg = 50/100 = 0.5 = Moderate: Vinyl

Ether

Cl/Cg = 1500/100 = 15 = High: Diethyl

Ether

Cl/Cg = 110000/100 = 1,100 = Very high:

Ethanol

Liquid = Water, T = 37OC

Substances Water/

Gas

Oil/Gas Blood/

Gas

Species

Acetone 395 86 245 Man

Benzene 2.8 492 7.8 Man

CarbonTetrachloride

0.25 361 2.4 Man

Cyclopropane 0.21 11.5 0.55 Man

Methyl EthylKetone

254 263 202 Man

SulfurHexafluoride

0.006 Man

Substances Saline/Gas Blood/Gas

1,1 dichloroethylene 0.4 5.0

Bromochloromethane 8 41

Diethyl Ether 11 12

Methyl ChloroformaFrom Gargas and Andersen,

1985

0.75 5.8